The human immunodeficiency virus (HIV), the causative agent of AIDS, is a complex retrovirus with levels of control imparted by regulatory and accessory proteins not found in simple retroviruses. it is important to continue to study the accessory proteins because they offer potential antiviral targets,because they teach us novel steps in HIV replication, and because they offer a window into control of normal host cell processes. In recent years it has also become clear that HIV accessory proteins serve as counter-measures to innate cellular antiviral mechanisms. This grant focuses on the interaction of the host cell with viral accessory proteins. In this proposal, we seek to understand the selective advantage that cell cycle arrest by the viralprotein Vpr confers on HIV. We will critically test the hypothesis that the selection for G2 arrest is influenced by cell cycle stage, virus production, and cell half-life using competition experiments and a smallmolecule inhibitor of Vpr function. We will also continue our study of the mechanism of cell cycle arrest by Vpr through its interaction with host cell proteins. In addition, Vpr interacts with a host cell DNA repair enzyme called Uracil DNA Glycosidase (UNG). Since UNG participates in the hypermutations induced by the Apobec3 proteins that are the target of the viral Vif protein, we will seek to understand the intersection of tbe Vpr and Vif pathways and will determine the role of UNG in the viral life cycle and the role of DNA repair in HIV hypermutation. Finally, we have begun to apply an evolutionary biology approach to understanding HIV host cell interactions by studying the positive selection of antiviral genes (the Apobec3 family) during primate evolution. This work will be extended by studying the evolution of other members of this family of antiviral genes, and by evaluating the evolution of Vif among the primate lentiviruses through phylogenetic and functional analysis of its adaptation to Apobec3 proteins in primates. It is anticipated that these: results will allow us to more fully understand the interaction of the virus with its host.